In recent years, desalination of seawater with composite semipermeable membranes has been started, and now water treatment plants are practically used for the desalination over the world. Composite semipermeable membranes generally have a composite membrane structure including a porous substrate film covered with a separating functional layer. When made of a crosslinked aromatic polyamide, the separating functional layer has the advantages that: it is highly stiff, because it contains benzene rings; it can be easily produced by interfacial polycondensation of an aromatic polyfunctional amine and an aromatic polyfunctional acid halide; and it can provide high salt removal ratio and high permeation flow rate (see Patent Literatures 1 and 2 below).
Today, high water quality standards are required of desalted water obtained by desalination with composite semipermeable membranes, and the need for the removal performance of composite semipermeable membranes becomes increasingly severe. In particular, the required level of performance for removal of boron, which is contained in a small amount in seawater, becomes increasingly severe.
However, desalination with conventional composite semipermeable membranes has difficulty in reducing the boron content to a level acceptable for drinking water.
Therefore, methods for improving the boron removal performance of composite semipermeable membranes have been proposed such as a method of treating a composite semipermeable membrane module with hot water (see Patent Literature 3 below) and a method of bringing the separating functional polyamide layer into contact with an aqueous bromine-containing free chlorine solution (see Patent Literature 4 below). However, when seawater having a temperature of 25° C., a pH of 6.5, a boron concentration of 5 ppm, and a TDS concentration of 3.5% by weight is subjected to a permeation process with these composite semipermeable membranes for desalination under an operation pressure of 5.5 MPa, the permeation flow rate through the membranes (membrane permeation flow rate) is at most 0.5 m3/m2/day, and the boron removal ratio is at most about 91 to 92%, which are still insufficient. Therefore, there has been a demand for development of composite semipermeable membranes with higher boron-rejection performance.
In order to improve the solute-rejection performance of composite semipermeable membranes, the pore size of the separating functional layer of composite semipermeable membranes may be reduced. In order to achieve sufficient permeability, however, the pore size should be moderately large. On the other hand, even through the pore size of the separating functional layer of composite semipermeable membranes is small, a relatively large number and amount of pores constituting composite semipermeable membranes can provide high permeability. In such a case, however, the solute-rejection performance tends to decrease. In order to improve solute-rejection performance and keep the permeability at a proper level, therefore, both of the pore size and the vacant content of the separating functional layer of composite semipermeable membranes have to be adjusted to appropriate levels.
In order to improve the performance of a composite semipermeable membrane including a porous substrate film and a separating functional polyamide layer formed thereon, investigations have been made on methods of moderately controlling both the pore size and the vacant content of the separating functional layer. A solution is a method that includes adding a new reactant to a reaction liquid to moderately control both the pore size and the vacant content. For example, there is proposed a method that includes allowing a polyamine component having two or more amino groups in the molecule to react with a new reactant, which is an acid component containing a linear aliphatic polyacid halide having two or more haloacyl groups in the molecule, to form a crosslinked polyamide (see Patent Literature 5 below). According to the literature, this method can be achieved by adding some modifications to conventional production methods and useful as a simple method for improvement and can produce a composite semipermeable membrane having high salt-rejection rate and high permeation flow rate. However, this method is still insufficient for increasing the boron-rejection rate to a satisfactory level.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 01-180208
Patent Literature 2: JP-A No. 02-115027
Patent Literature 3: JP-A No. 11-19493
Patent Literature 4: JP-A No. 2001-259388
Patent Literature 5: Japanese Patent No. 3031763